Cavities and Magnets Working Group Darin Kinion (LLNL) 4/26/2012

G. Rybka Vistas in Axion Physics Cavity Axion Searches Asztalos et al. PRL 104, (2009) Cavity experiments are sensitive to axions in the range 1 μev – 100 μeV ADMX experiment a B γ

G. Rybka Vistas in Axion Physics Axion Search Big Picture Source: T. Dafni, PATRAS 2010 (modified)

Big Questions: Should we still be looking for axions? – Yes! Should we be using microwave cavities to search for axions? – Yes! What mass (frequency) range should be to goal? – No overwhelming theoretical consensus – Stick to “natural” range for existing cavity amplifier technology (100 MHz – 40 GHz)

Power from Axion-Photon Conversion B = Magnetic Field Strength V = Volume of cavity(ies) Q = min{Q L,Q a } C mn = Form factor

Stored Energy (B 2 V) NHMFL – very interesting array of large volume, high-field magnets Built magnets could possibly be utilized, but no natural fits for ADMX New magnets – very expensive ($7M-$50M) Retrofit, renovation of existing magnet could be problematic

Cavity Q ADMX-HF exploring the idea of using superconducting thin films to reduce losses in cavity walls and tuning rods Requires homogenous B field to reduce radial component Factor of 6 improvement possible

System noise temperature Combination of physical temperature and first amplifier (mostly) noise temperature Superconducting amplifiers provide near quantum limited performance up to ~ 8-10 GHz HFETs above 8-10 GHz, pending future amplifier developments Quantum Noise ~ hf/kb – above 6 GHz reduce need for dilution refrigerators (He3 systems)

Cavity form factor Drives choice of mode typically TM 010 for the right-circular cavity Higher modes provide path to higher frequencies, as well as in situ testbed for new amplifiers Extensive use of Finite Element software

Push to higher frequency For ADMX, r = 21 cm  f = 550 MHz L = 100 cm Or:

Length cannot get too long The longer the cavity, the more TE modes there are in the tuning range. With metal tuning rod, there are also TEM modes at ~ integer*c/2L ~ 150 MHz for 1 m L Typical values L ~ 5r = 2.5*diameter Modes for r = 3.6 cm, L = 15.2 cm cavity. d is the distance the metal rod is from the center. (Divide frequencies by 6 for ADMX.)

Push to higher frequencies As the cavity(ies) get smaller the question becomes what to fill the remaining magnet volume with – Multiple cavities – Small cavities with TM010-like modes – More magnet wire (increase B0 as volume shrinks)

ADMX operated a 4 cavity array Did not fill the cavity volume well

Cavities must operate at the same frequency

Segmented Resonator ~¼ scale prototype – TM010 frequency = 2.7 GHz – Q  25,000 (300K) – V  5 liters Scaled to ADMX, would have f = 850 MHz 4 segment resonator would have f = 1.1 GHz

Need up to 32 cavities Covers about 1 decade in axion mass

Detecting higher axion masses Higher frequency resonant structures f res ~ 10 x f 0 ~ 3 GHz

Yale experiment- single small cavity Cu resonant cavity at 34 GHz, cooled to T=4 K, tunable, TE 011 mode. 18Vistas in Axion Physics 2012

Summary Current searches are underway – ADMX & ADMX-HF – Yale search at 30+GHz Incremental improvement possible in B,V but very expensive Factor of 6 possible in Q Strategy for covering higher frequencies is a real issue